Note: if using unsalted cashew nuts, add 1/8 teaspoon salt with the
sugar

Using vegetable oil, generously oil a sheet
pan (preferably one with sides), at least 11 by 17 inches. In a
medium-size heavy saucepan, combine the water, sugar, cream of
tartar and corn syrup and bring to a boil over medium heat. Using a
candy thermometer to test it, boil the mixture until it reaches
350ºF, or is amber-colored.

Remove from the heat and, working quickly, whisk in the cinnamon.
Whisk in the butter, cashews, and baking soda. Pour the mixture onto
the oiled pan and spread it out a bit with a wooden spoon, to about
1/4 to 1/2-inch thickness. Don't spread it too thinly.

Let harden, uncovered, in a cool place, 30 to 45
minutes. (To wash the saucepan, soak it overnight.) Using your
hands, break the brittle into pieces. Store in an airtight
container.